1. Field of the Invention
The invention relates to technology of an electrode paste manufacturing method, and a manufacturing system for realizing the manufacturing method, and a secondary battery manufactured using the electrode paste manufactured according to the manufacturing method.
2. Description of Related Art
Technology for producing an electrode paste using a twin shaft extrusion mixer in order to produce good quality (i.e., homogeneous) electrode paste is known. This technology is described in Japanese Patent Application Publication No. 2011-224435 (JP 2011-224435 A), for example, and is well known.
JP 2011-224435 A describes a mixer (i.e., a twin screw extrusion mixer) that is equipped with a hollow barrel and two rotating shafts provided parallel a predetermined distance apart from one another in a mixing chamber formed inside the barrel. A binder injecting portion that is supplied with binder is arranged downstream, in a powder transfer direction, of a powder injecting portion that is supplied with powder, in the mixing chamber of the mixer. A powder processing portion that is equipped with a spacer that is provided on the rotating shafts and compresses the powder is arranged between the powder injecting portion and the binding injecting portion. A good quality (i.e., homogeneous) paste is able to be produced by producing an electrode mixture (i.e., an electrode paste) using this kind of twin screw extrusion mixer.
When manufacturing electrode paste using a twin screw extrusion mixer such as that described in JP 2011-224435 A, the inside of the twin screw extrusion mixer is not airtight (i.e., it is an open system), so all of the produced electrode paste that is discharged from the twin screw extrusion mixer must first be received by a tank and then be batch processed before it is introduced into a closed system (i.e., a deaerating tank) for vacuum deaeration. Thus, when producing electrode paste using a twin screw extrusion mixer, the production time is increased by the amount of time required for the batch processing, so when using a twin screw extrusion mixer, it is difficult to shorten the time that it takes to produce electrode paste.
Also, in recent years, various technologies for reducing the amount of solvent in electrode paste have been examined in attempt to reduce the manufacturing cost of secondary batteries or reduce the environmental load and the like.
With electrode paste, if bubbles are mixed into the paste, they may cause problems (transparency) when a coat of the electrode paste is applied. Therefore, a vacuum deaerating process is performed on the electrode paste to remove the bubbles in the electrode paste. However, if the amount of solvent is reduced and the solid fraction of the electrode paste increases, it is difficult to remove the bubbles, which is problematic. For example, if the amount of solvent is reduced and the solid fraction of the electrode paste is 50% or more, the viscosity of the electrode paste increases. Also, when the viscosity of the electrode paste is high, it is difficult to remove the bubbles in the electrode paste.
Therefore, when using a high viscosity electrode paste in which the amount of solvent has been reduced, even after the vacuum deaerating process is performed, a supernatant (i.e., top) portion that includes a large amount of bubbles is not used, only the portion with few bubbles is used. Therefore, the yield of the electrode paste ends up being poor, so manufacturing cost of the electrode paste has been unable to be reduced as hoped.